1. Field of the Invention
The present invention relates to a heat pipe for transporting heat as latent heat of a working fluid such as a condensable fluid, and especially to a heat pipe which is constructed to create a so-called pumping force for refluxing a liquid phase working fluid to a portion where it evaporates, by means of a capillary pressure of a porous material.
The present invention relates to the subject matter contained in Japanese Patent Application No.2003-38404, filed on Feb. 17, 2003, which is expressly incorporated herein by reference.
2. Related Art
In the customary way, a heat pipe for transporting heat in the form of latent heat of a working fluid is well known in the prior art. The heat pipe of this kind is a heat conducting element encapsulating a condensable fluid such as water in a sealed receptacle (container) after evacuating an air therefrom, and which is constructed to transport the heat as latent heat of a working fluid by evaporating the working fluid with the heat inputted from outside, and by condensing a vapor by radiating the heat after the vapor flows to a condensing part of a low temperature and a low pressure. Accordingly, since the heat is transported in the form of latent heat of the working fluid, the heat pipe has more than ten times to several hundred times of heat transporting capacity in comparison with that of copper which is known to have the highest heat conductivity.
According to the heat pipe of this kind, the heat is transported by means of flowing the evaporated vapor phase working fluid to the condensing part in the low temperature and low pressure side, and after the heat transportation, the condensed liquid phase working fluid is refluxed to the evaporating part (i.e., a heat inputting part) by a capillary pressure of a wick.
The wick is, in short, a member for creating a capillary pressure, and therefore, it is preferable to be excellent in so-called hydrophilicity with the working fluid, and it is preferable to have its effective radius of a capillary tube as small as possible at a meniscus formed on a liquid surface of the liquid phase working fluid. In this connection, a porous sintered compact or a bundle of extremely thin wires is employed as a wick generally in the customary way. Among those wick members according to the prior art, the porous sintered compact may create great capillary pressure i.e., a pumping force to the liquid phase working fluid, because the opening dimensions of its cavities are smaller than that of other wicks. Also, the porous sintered compact may be formed into a seat shape so that it may be employed easily on a flat plate type heat pipe or the like called as a vapor chamber, which has been attracting attention in recent days. Accordingly, the porous sintered compact is a preferable wick material in light of those points of view.
The heat transporting characteristics of the heat pipe including the vapor chamber is thus improved as a result of an improvement of a wick material and so on, and miniaturization is also attempted in connection with this. At the same time, how to cool a personal computer, a server, or a portable electronics device, which are enhanced in its compactness and capacity, has been becoming a problem in recent days. The heat pipe has been garnering the attention as a means for solving this problem, and it has been employed more frequently. Examples of employing such downsized and thin-shaped heat pipe are disclosed in Japanese Patent Nos. 2,794,154 and 3,067,399.
As described above, it is possible to increase the capillary pressure for refluxing the liquid phase working fluid if a porous body is employed as a wick to be built into the heat pipe. This is advantageous for downsizing the heat pipe (or the vapor chamber). If the liquid phase working fluid is refluxed by utilizing the pumping force of the capillary pressure, the liquid phase working fluid is carried inside of the wick; however, in case of the wick of a porous body, because a flow path created therein is the cavity created among the fine powders as the material of a porous body, so that the flow cross-sectional area of the flow path has to be small and as intricate as a maze. Therefore, there is a disadvantage in that the flow resistance is relatively big. Also, the liquid phase working fluid is to be contained in the cavity so that an amount of the working fluid is not always sufficient. Accordingly, if the inputted amount of heat from outside increases suddenly and drastically, for example, there will be a possibility of so-called drying out such that the wick goes into a dry state due to a shortage of the liquid phase working fluid fed to the portion where the evaporation of the working fluid takes place.
Moreover, in general, the porous body to be employed as the wick is produced by sintering the fine powder material, so that there is no particular bias on a void content and it is uniformally even. If the wick of the porous body is moistened by the working fluid, the liquid phase working fluid disperses almost uniformly over the entire part of the wick. Since this is likewise exemplified even when the heat pipe is under operation, the liquid phase working fluid is dispersed and contained even in the portion where the heat is not inputted from outside, in case of the vapor chamber wherein the sheet-shaped porous body is employed as the wick. Consequently, this causes a reduction of the reflux rate or feeding amount of the liquid phase working fluid, to the portion where the heat is inputted from outside. Accordingly, there is room for improvement from this point of view.